Shoot suppression is important in image detail enhancement process in order to reduce undesirable artifacts introduced into the enhanced image. FIG. 1 shows a system block diagram for a conventional image detail enhancement system 10 without shoot suppression. Such a system is typically known as “unsharp” filter, wherein an input signal f (representing at least a portion of a video image formed by pixels), is processed and output as a signal g. The signal f is provided to a low pass filter (LPF) 12 to generate a low-passed signal f1. Then the difference between the signals f and f1 is determined in a difference junction/node 14, as a detail (difference) signal, (f−f1). The detail signal is then multiplied by a constant K (K>1) for enhancement in a multiplication junction/node 16, and summed with the signal f1 in a summing junction/node 18, to generate the output signal g. As such, the relationship between the output signal g and the input signal f can be expressed in the following equation:g=(f−f1)*K+f1  (1)
Equation (1) can also be expressed as:g=(f−f1)*(K−1)+f  (2)
Therefore, an equivalent form of the system block diagram can be as shown in FIG. 2. In equation (2), the product (f−f1)*(K−1) is the detail enhancement term. Therefore, the details come from the detail signal (f−f1). Wherever there is a non-zero difference between the signals f and f1 there are details which may be enhanced in the detail enhancement process.
However, in an actual image, not all the areas with a difference between the signals f and f1 are desirable for enhancement. An example is shown in FIGS. 3a-b, wherein FIG. 3a is the original image and FIG. 3b is the detail-enhanced image without overshoot/undershoot suppression. As can be seen, undesirable artifacts are introduced around edge areas in the enhanced image in FIG. 3b. This is known as overshoot/undershoot, and the arrows in FIG. 3b indicate some of the “shoot areas”.
In order to remove such undesirable artifacts from detail-enhanced images, a shoot suppression mechanism is necessary. The system block diagram in FIG. 4 shows an example of such a mechanism. The system includes a shoot suppression block 15, wherein the output of the shoot suppression block 15 is a shoot suppression factor s, wherein 0≦s≦1. The lower the value of s, the higher the suppression provided. The suppression factor is applied to the detail signal (f−f1), forming a detail enhancement term, and the result is combined with the input signal f to generate the detail-enhanced output signal g. The relationship between the output signal g and the input signal f in FIG. 4 can be expressed as:g=(f−f1)*(K−1)*s+f  (3)
Several methods and systems exist that attempt to suppress the undesirable shoot artifacts. However, many such shoot suppression methods also degrade the overall enhancement quality by suppressing the enhancement in non-shoot areas. As a result, shoot artifacts are suppressed, but the resulting image is not much enhanced either. There is, therefore, a need for a shoot suppression method and apparatus that provides suppression only in shoot areas while maintaining good enhancement in non-shoot areas.